1. Field of the Invention
This invention relates to a radiation detector, a radiation detecting method and an X-ray diagnosing apparatus provided with the same radiation detector, and more particularly to an art for improving fluoroscopy image quality and reducing signal noise.
2. Description of the Prior Art
Conventionally, as an image detector, an X-ray fluoroscopy/radiography apparatus provided with an image intensifier (I.I)-TV camera system has been used.
In cardiovascular inspection with an X-ray fluoroscopy/radiography apparatus provided with I.I-TV system, panning of an imaging object is accompanied and fluoroscopy is carried out for a portion in which an X-ray low absorption region such as lung field, diaphragm and the like and an X-ray high absorption region including the heart exist mixedly. To prevent halation in the X-ray low absorption region and improve fluoroscopy image quality, conventionally, such mechanical filters as an electric collimator, electric compensation filter and the like have been used. However, an operation of the mechanical filter must be carried out each time when an image angle is changed and thus the operation is very complicated.
Therefore, as a method for improving the fluoroscopy image quality, recently "Automatic Brightness Correcting Method for X-ray Imaging Apparatus" (Japanese Patent Application Laid-Open No. 4-32993), and "Dynamic Compression Method for X-ray Recording and Apparatus therefor" (Japanese Patent Application Laid-Open No. 4-271677) have been proposed.
Further, recently, as an X-ray solid flat panel detector using semiconductor production technology, an indirect conversion type X-ray solid flat panel detector (U.S. Pat. No. 4,689,487, etc.) and direct conversion type X-ray solid flat panel detector (U.S. Pat. No. 5,319,206, etc.) have been proposed.
In this indirect conversion type X-ray solid flat panel detector, the X-ray is converted to visible light through an intensifying paper or a chemical substance like cesium iodide (CsI), an intensity of this visible light is converted to electric charge by photoelectric conversion action of a photo diode and this electric charge is accumulated in a capacitance of each picture element.
Then, accumulated electric charge is successively read out by a switching means like a thin film transistor (hereinafter referred to as TFT) matrix and converted to a voltage by a charge amplifier (called initial stage integration amplifier also). Then, this voltage is analog/digital converted so as to obtain a digital image signal (see FIG. 1).
On the other hand, in the direct conversion type X-ray solid flat panel detector, the X-ray impinging upon a semiconductor like a selenium (Se) under a high electric field contributes to generation of electric charge by direct photoelectric effect and this electric charge is accumulated in a signal accumulated capacitance of each picture element. FIG. 2 shows a schematic sectional view thereof. Like the indirect conversion type, the accumulated electric charge is read out successively by switching of the TFT, converted to a voltage by an integration amplifier (not shown) and further analog-digital converted so as to obtain a digital image signal.
FIG. 3B shows changes of the number of electrons generated per unit absorption energy when the bias voltage Vb is changed between 1000 V and 1700 V in amorphous-selenium (a-Se) having a film thickness of d.sub.se =160 .mu.m shown in FIG. 3A. (for reference: "X-ray imaging using amorphous selenium: Determination of X-ray sensitivity by pulse height spectroscopy", J. A. Rowlands, G. DeCrescenzo, and N. Araj, Medical Physics VOL. 19, No. 4, pp 1065-1069, July/August 1992).
However, with respect to X-ray imaging apparatus using method, X-ray condition, namely, an X-ray amount (mAs) which is a product of X-ray tube current and exposure time differs largely between fluoroscopy and radiography, and an amount of electric charge accumulated in each picture element of the X-ray solid flat panel detector differs in each condition.
For example, there is a difference several hundreds times in terms of maximum electric charge amount accumulated in a single picture element, between fluoroscopy and radiography conditions. Therefore, in a signal range like fluoroscopy condition, having a small amount of electric charge, a digital data valid maximum range of an A/D converter (hereinafter referred to as ADC) cannot be sufficiently utilized, so that its signal resolution after the digital conversion drops, which is a problem of the conventional art.
In the conventional fluoroscopy image quality improving method, if light intensity from the I.I is over a dynamic range of a camera, there is no way for improvement because this method is a method for applying various processings to a video signal after an output of the image intensifier is taken by a TV camera.
On the other hand, in a conventional X-ray solid flat panel detector, a relation of an output signal with respect to input X-ray is almost linear and uniform in terms of a plane of a flat panel and this relation cannot be changed depending on a two-dimensional region of an detection plane.
Further, the conventional X-ray solid flat panel detector has the following problem about signal noise.
FIG. 4 is a diagram showing a picture element circuit composed of photoelectric elements and switching elements disposed in the matrix. The picture element is indicated by a capacitor Cp for accumulating electric charge as an equivalent circuit. If the X-ray is irradiated, the electric charge Q accumulated in the picture element Cp is sent to a signal line 112 at a timing in which a switching element 111 like TFT is turned on. The electric charge sent to the signal line 112 is converted to a voltage Vo by an integration amplifier (integration circuit comprising an operational amplifier 131 and a capacitor Cf) and this voltage Vo is accumulated and sample held in the capacitor C by turning on a switch 141 for sampling of a sample/hold circuit 114.
It has been known that noise superimposed on the output voltage Vo of the integration amplifier 113 is given in a formula below and this is described in a marketed data sheet. EQU noise=(noise caused in the integration amplifier).times.[1+(Cs/Cf)](1)
Here, Cs is a capacitance component from a switching element up to input of the integration amplifier 113 and includes a stray capacitance of the signal line 112. Cf is a capacitance component for supplying a relation between output voltage Vo of the integration amplifier and input electric charge Q and is given in the form of Vo=-Q/Cf.
As evident from the formula (1), in the conventional circuit, it is necessary to improve the performance of the integration amplifier 113 and reduce Cs in order to reduce noise.
However, not only the Cs cannot be reduced to zero but also it increases if the signal line 112 for carrying the electric charge Q of each picture element Cp disposed in the matrix is extended, so that there is a limit in reduction of noise. Further, even if Cs is designed so as to be small enough, the Cs cannot be reduced below noise component of the integration amplifier 113. Then, that noise is automatically determined by the performance of the FET on an initial stage composing the integration amplifier 113. The FET performance also has a limit and a low noise FET is expensive and difficult to mass produce.
A countermeasure shown in FIG. 5 for the above condition has been described in "CMOS low noise amplifier for microstrip readout design and results", Nuclear Instruments and Method in Physics Research A301 (1991).
Referring to FIG. 5, a shaping circuit 115 for noise reduction surrounded by broken line is inserted after the integration amplifier 113. Consequently, an interval between the integration amplifier 113 and sample/hold circuit 114 has following frequency characteristic. EQU F(.omega.)=a.multidot..omega./SQR(.omega..sup.2 +b.sup.2) (2)
SQR indicates a root sign, and a=Cd/Cx, b=1/(Cx-Rx) and .omega. is an angular frequency component of a signal to be input to this circuit.
According to this formula (2), noise in low frequency component can be removed by the shaping circuit 115, noise of sampling time is reduced. Further, the same reference describes a circuit in which an integrator comprising a capacitor and a resistor is provided after the shaping circuit 115 and by providing with such an integrator, there is an effect that even high frequency noise can be removed.
The circuit shown in FIG. 5 is designed to successively detect electric charge caused by radiation arriving in the form of pulse like in a gamma radiation detector. As shown in FIG. 6A, the wave shape of the output voltage Vx of the shaping circuit 115 becomes pulse-like. At this time, the output signal Vo of the integration amplifier 113 is as shown in FIG. 6B and damps with a passage of time. A position of a pulse signal shown in FIG. 6A depends on a resistance existing on the signal line 112. Further, its peak value is determined by this resistance and electric charge amount Q which is a signal source. Therefore, if the resistance is not changed, the peak value is preliminarily obtained and sampling is carried out at that position. Then, a signal corresponding to the electric charge Q is sampled.
However, as the switching element 111 for taking out electric charge from picture elements disposed in the matrix, a type in which the resistance like TFT is changed by the electric charge is sometimes used. In such a case, a relation between voltage (Vds) and current (Ids) does not become linear as shown in FIG. 7. This is a characteristic generally observed in the switching element 111 having the transistor structure.
Because of this characteristic, if the circuit shown in FIG. 5 is used, sampling at a constant timing cannot be carried out because the peak position of the output voltage Vx shown in FIG. 6A changes depending on the electric charge Q. Further, the relation between the peak value and electric charge Q becomes very complicated, so that the circuit shown in FIG. 5 cannot be used as it is.